Electrostatic recording element



United States Patent 3,346,381 ELECTROSTATIC RECORDING ELEMENT Harold G. Greig, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed July 30, 1964, Ser. No. 386,407 8 Claims. (Cl. 961.8)

This invention relates to improvements in electrostatic printing and copying. More particularly the invention relates to improved coatings for electrostatic recording elements, especially electrostatic recording elements which are to be processed in apparatus utilizing a liquid developer.

By an electrostatic printing process is meant generally a process for producing a visible image, reproduction, or copy which includes, as an intermediate step, converting a light image or signal into an electrostatic charge pattern on a recording element comprising a photoconductive insulating coating on a carrier base. The process includes converting the charge pattern or latent image into a visible image which is a substantially faithful reproduction of an original. The latent image is developed by ap plying a toner material, carrying an electrostatic charge. The toner material is attracted and held by the oppositely charged image areas, and fixed in place, for instance by melting the toner material so that it fuses to the surface, to produce a durable, light-East image. Such a process and recording element are described in United States Patent 3,052,539, issued Sept. 4, 1962.

One of the ways of applying the charged powder to the image areas is by use of a liquid developer. A liquid developer generally contains toner material dispersed in an inert organic liquid of high volatility that is substantially nonflammable, noncorrosive, etc. One of the organic liquids used for liquid developers is trifiuoro trichloro ethane, which is available commercially as Freon 113 or Freon TF.

Many electrostatic coatings are softened and the electrical properties adversely affected when exposed to liquid carriers such as Freon TF, so that the electrostatic latent 40 image is weakened or lost entirely. Cellulose acetate 3,346,381 Patented Oct. 10, 1967 It is a further object of this invention to provide improved electrostatic recording elements which are stable under high humidity conditions.

It is another object of the present invention to provide electrostatic recording elements which resist softening when immersed in liquid developers.

It is a particular object of this invention to provide a recording element utilizing cellulose acetate butyrate as a binder, which is stable under conditions of high humidity.

An electrostatic coating composition that accomplishes the objects of this invention may be prepared using zinc oxide as a photoconductive material, cellulose acetate butyrate resin as a binder, and mono-organo silsesquioxane :as a stabilizer to prevent image and coating deterioration due to moisture. The molecular configuration of the mono-organo silsesquioxane may be described by the following formula:

where R indicates an organic group, principally phenyl 'but including prophyl groups. As a theory of operation, it is believed that the mono-organo silsesquioxane blocks the hydroxyl groups present in cellulose acetate butyrate, making it less hydrophilic and thereby increasing the compatibility of the coating with a high humidity environment. A high humidity environment is one having a relative humidity of '80 to percent. Mono-organo silsesquioxane corresponding to the foregoing description is available commercially from the Dow Corning Corporation under the trade name Z6018.

Compositions and properties of typical cellulose acetate butyrates are shown in Table I.

TABLE I Composition and Properties 1 AB-1612 I AB-272-3 AB-38l-l I AB-500-1 l AB5005 Acetyl, percent Butyryl, percent Hydroxyl, percent" Group per Cs unit, percent H 95% RH 0. 8 O. 9 0. 6 0. 4 O. 5 1. 7 1. 8 1. 1 0. 6 0. 6 2. 8 3. 3 1. 8 1. 1 1. 1 4. 4 6. O 3. 0 2. 0 2. 4

butyrate is used as a binder in some recording elements because it is resistant to degradation by trifiuoro trichloro ethane. However, coatings having a cellulose acetate butyrate binder are sensitive to moisture and tend to deteriorate when the carrier base contains moderate amounts of moisture.

It is an object of this invention to provide improved recordingelements for use in electrostatic printing.

The source of the data for Table I is Payne, Organic Coating Technology, Vol. 1, Wiley and Sons, New York, 1954, page 447.

Ideally, the cellulose acetate butyrate would have no hydroxyl content and would be selected for that purpose. However, various cellulose acetate butyrates have properties which impart desirable properties into the coating, and therefore the hydroxyl content should not be the sole consideration in the selection of the butyrate. Also, the commercially available cellulose acetate butyrates have some measurable hydroxyl content. The amount of mono-organo silsesquioxane used is directly related to the hydroxyl content of the cellulose acetate butyrate.

Formulation of electrostatic coating mixes have been made with several different cellulose acetate butyrate resins, basing the added amount of mono-organo silsesquioxane upon the hydroxyl content of the particular cellulose acetate butyrate used. All cellulose acetate butyrate resins have been found to be operative in combination with the mono-organo silsesquixoane. However, the resins of the highest acetyl and butyryl content are desired. As a theory of operations, it is believed that the higher acetyl and butyryl contents result in an increase in the irregularity of the substitution of the polymer chain to spread the chains farther apart. Increased butyryl content of the ester lowers the polar character of the resin and increases its solubility and compatibility with other insulating materials. Another desirable feature is that the bulky character of such resins produces an electrostatic coating having a good appearance and a hand of paper. That is, the coated paper feels like paper without a coating.

The cellulose acetate butyrate resin modified with the mono-organo silsesquioxane increases the electrical resistivity and, with suitable additives, imparts desired physical characteristics to an electrostatic recording element coating. The resins are versatile, relatively low cost binders for zinc oxide in' electrostatic coatings.

Other modifiers can be added to obtain the physical and electrical characteristics needed in an electrostatic coating. T riphenyl phosphate can be used as a plasticizer. Sucrose acetate isobutyrate can be used to increase the adhesion of the coating to a substrate material, lower the viscosity of the mix and improve the electrical characteristics of the coating. The sucrose acetate isobutyrate has an extremely high viscosity index (semi-solid at room temperature, 1000 centiposies at 70 C., and 100 centiposies at 100 C.), which can be useful in adapting coating mixes to various coating procedures. Sucrose acetate isobutyrate has excellent stability to light and is efiicient electrically, having a volume resistivity of 6.4x ohm-cm. Also, during the fusing step in which the powder is fixed to the substrate to make permanent the electrostatic image, the sucrose acetate isobutyrate reduces the curl of the paper. That is, it aids the paper to flatten out again. Various ingredients may be added to adjust the viscosity of the mix for coating purposes such as silicone fluid (General Electric SF-69).

In preparing a coating composition, the components other than zinc oxide are dissolved in an organic solvent. The Zinc oxide is then added to the solution and thoroughly dispersed. Suitable organic solvents are ketones, esters, alcohols, aromatic hydrocarbons, chlorinated solvents and solvent blends having kauri-butanol values greater than 50. Particular examples are: acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl acetate, ethyl acetate, isopropyl acetate, n-butyl acetate, methylene chloride, ethylene chloride, propylene chloride, nitromethane, nitroethane, 2-nitropropane, methyl Cellosolve acetate, Cellosolve, ethyl Cellosolve, 1,4-dioxane, tetrachloroethane, cyclohexanone, etc. The coatings may be applied to a base material by the knife-coat, spray, dip, gel-dip, hot-melt and other well known methods. The base material may be either a relatively good conductor, such as metal, or a relatively poor conductor, such as a cellulosic sheet, for example, paper or cellophane.

The nature of the present invention has been described. Specfic examples of the practice of the invention are hereinafter given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims.

4 Example I A coating mix was prepared using the following materials:

Material: Parts by weight Cellulose acetate butyrate AB-500-1 -a 150 Mono-organo silsesquioxane 25 Sucrose acetate isobutyrate 25 Triphenyl phosphate 25 Methyl ethyl ketone 1500 Example II Material: Parts by weight Cellulose acetate butyrate AB-272-3 150 Mono-organo silsesquioxane Sucrose acetate isobutyrate (100%) 25 Triphenyl phosphate 25 Methyl ethyl ketone 1500 Zinc oxide 705 The procedure of Example I was repeated except that cellulose acetate butyrate AB-272-3 was substituted for the cellulose acetate butyrate AB-500-1. The AB-500-1 has 0.5 percent by weight hydroxyl content while the AB- 272-3 has a 2.5 percent by weight hydroxyl content. It was found necessary to use 125 parts by weight of monoorgano silsesquioxane instead of 25 parts by weight as .in Example I in order to make a satisfactory coating for printing in an electrostatic process at high humidity.

As a control, the same formulation was used without the silsesquioxane and produced no printing when coated on a carrier base and used in an electrostatic process.

The addition of the silsesquioxane resulted in a coating composition that, after application to a carrier base and electrostatic processing, produced a commercial quality print.

Example 111 Material: Parts by weight Cellulose acetate butyrate (EAB-V seconds) 1 30 Mono-organo silsesquioxane 6 T riphenyl phosphate 5 Zinc oxide Photox 801) Methyl ethyl ketone 200 1 One-tenth of a second is the time taken for a 1 inch steel ball to fall through ten inches of a 20% solution of the cellulose acetate butyrate in a 90/10 mixture of acetone/2B denatured alcohol at 25 C. in a. 1-inch tube.

The coating mix was prepared as in Example I and gave excellent coatings. Prints were obtained as in Example I even after several days storage at high humidity.

The terms and expressions which have been employed are used as terms of description and not limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. Various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A recording element for electrostatic printing comprising a carrier base and a photoconducting insulating layer attached to said base, said layer being comprised of zinc oxide, cellulose acetate butyrate and mono-organo silsesquioxane.

2. A recording element for electrostatic printing comprising a base material carrying a photoconducting layer comprising zinc oxide suspended in an electrically insulating film-forming vehicle, said film-forming vehicle further comprising cellulose acetate butyrate and mono-organo silsesquioxane.

3. A recording element for electrostatic printing comprising a base material carrying a photoconductive insulating layer including zinc oxide, cellulose acetate butyrate, and a stabilizer having the chemical structural formula:

R R SLM'I s1 0 Si 0% o J I 0 \OH Sr-O-Sr it 1'; where R is largely phenyl groups, the balance being proide, about parts by weight of cellulose acetate butyrate, and about 25-125 parts by weight of mono-organo silsesquioxane, said recording element being stable under conditions of high humidity.

6. A recording element for use in an electrostatic printing process in which trifluoro trichloro ethane is used as a dispersion medium for toner particles in a liquid development step comprising a carrier base coated with a composition comprising zinc oxide, cellulose acetate butyrate having a hydroxyl content between about 0.5 and 2.5 percent by weight and sufficient mono-organo silsesquioxane to block the hydroyl content of said cellulose acetate butyrate and render the material substantially hydrophobic, whereby the recording element is stable in an environment having high humidity.

7. A process of forming an image comprising providing a recording element comprising a base material carrying a composition comprising zinc oxide, cellulose acetate butyrate, and mono-organo silsesquioxane; forming a pattern of electrostatic charges corresponding to an image on a surface of said composition; and developing said pattern by contacting said surface with a liquid developer containing a finely divided toner material, which selectively deposits on the surface bearing said pat-tern in configurations conforming with said pattern, whereby a high quality reprodution is produced which is stable in a high humidity environment.

8. An electrophotographic process comprising the steps of imposing an electrostatic charge on a photoconductive insulating layer comprising zinc oxide, cellulose acetate butyrate, and mono-organo silsesquioxane on a conductive carrier base; exposing the charged surface to a light image to produce an electrostatic latent image; developing the electrostatic latent image by contacting said surface layer with an organic developer made up of toner particles dispersed in trifluoro trichloro ethane; and fixing the image, whereby a high quality reproducstion is produced which is stable and free from deterioration under conditions of high humidity.

References Cited UNITED STATES PATENTS 3,155,504 11/1964 Damm et al 961.8

NORMAN G. TORCHIN, Primary Examiner. 

1. A RECORDING ELEMENT FOR ELECTROSTATIC PRINTING COMPRISING A CARRIER BASE AND A PHOTOCONDUCTING INSULATING LAYER ATTACHED TO SAID BASE, SAID LAYER BEING COMPRISED OF ZINC OXIDE, CELLULOSE ACETATE BUTYRATE AN MONO-ORGANO SILSEQUIOXANE. 